Data communication occurs as the transfer of information from one communication device to another. In some devices, data communications are accomplished by the use of a modem located at each communication endpoint. In the past, the term “modem” denoted a piece of communication apparatus that performed a modulation and demodulation function, hence the term modem. Today, the term modem is typically used to denote any piece of communication apparatus that enables the transfer of data and voice information from one location to another. For example, modern communication systems use many different technologies to perform the transfer of information from one location to another.
Digital subscriber line (DSL) technology is one vehicle for such transfer of information. DSL technology uses the widely available subscriber loop, the copper wire pair that extends from a telephone company central office to a residential location, over which communication services, including the exchange of voice and data, may be provisioned. DSL devices can be referred to as modems, or, more accurately, transceivers, which connect the telephone company central office to the user, or remote location typically, referred to as the customer premises. DSL communication devices utilize different types of modulation schemes and achieve widely varying communication rates. However, even the slowest DSL communications devices achieve data rates far in excess of conventional point-to-point modems.
DSL transceivers can be used to provision a variety of communication services using, for example, but not limited to, asynchronous transfer mode (ATM). ATM defines a communication protocol in which data cells are used to carry information over the DSL communication channel. The first portion of the ATM cell is typically used for overhead and the remaining portion is used to carry the communicated information or data. When using a switched-carrier transmission methodology, a control transceiver may be connected via the DSL to one or more remote transceivers. In such a communication scheme, the transmission is commonly referred to as “half-duplex,” which is defined as two way electronic communication that takes place in only one direction at a time. With only a single remote transceiver on a line, switched-carrier transmission may instead be employed in full-duplex mode (allowing transmission in both directions simultaneously). In this case, full-duplex operation is typically enabled by employing either echo cancellation or frequency division multiplexing. Hybrid techniques are possible such as one in which there are multiple remote transceivers and communication takes place between the control transceiver and only one remote transceiver in full-duplex fashion.
Before the transmission of ATM cells, a preamble containing channel, transmission, address and administrative information may be transmitted by the transceiver. The application of this preamble is sometimes referred to as “framing” the data to be transmitted. The resulting sequence of symbols assembled for transmission, including preamble and (possibly) cells, is then referred to as a “frame”. Due to the switched-carrier nature of the transmission, silence precedes this preamble. It is desirable to have the ability to precisely delimit the beginning and end of a transmission to within one transmitted symbol interval. Robustly delimiting the beginning of a frame enables a receiving transceiver to reliably begin decoding the frame at the correct symbol. Likewise, robustly delimiting at the end of a frame enables a receiving transceiver to reliably decode the entire frame through the final symbol, and then stopping so as to prevent data loss and to prevent the inclusion of any false data. Furthermore, by communicating an end of frame indicator to a receiving transceiver prior to the actual end of the frame, line turnaround time (ie., idle time on the line between transmissions) can be reduced, thereby increasing the effective use of the available line bandwidth.
Because the most efficient signal constellation encoding cannot allocate signal space to silence, it is impractical to reliably discriminate silence from a signal when analyzing only a single symbol encoding an arbitrary data value. To improve frame delimiting, existing techniques use special marker symbols whose symbol indices are greater than those used to encode data. At N bits per symbol (bps), data is encoded using symbol indices 0 through 2N-1. The special symbols use indices 2N and above. While these special marker symbols are useful for marking the beginning and end of a transmission, their placement at the outer edges of a constellation raises the peak signal, thus increasing the peak to average ratio (PAR) across all data rates by as much as 4 dB. Unfortunately, discrimination of special symbols has the same error threshold as does decoding of data.
DSL systems typically operate in the presence of crosstalk from other DSL systems in the same loop plant (a communication system having a plurality of subscriber loops, where portions of the subscriber loops share a common location and are in close proximity to each other). Crosstalk is generated when a signal transmitted on one pair of wires couples electromagnetically to a nearby pair of wires to create an unwanted signal which is received together with the intended signal. DSI, systems use a variety of techniques to adapt to crosstalk, as well as the distortion introduced by the loop and other impairments. In order for a receiver residing in the transceiver to adapt to crosstalk, the crosstalk must be detectable.
DSL systems which use burst transmission are generally known as Short-Term Stationary (STS) systems. DSL systems are characterized by two distinct transmission states. An STS transceiver alternates between an ON state, in which a signal is transmitted, and an OFF state, in which either silence or a lower power and/or lower bandwidth signal is transmitted. The duration of individual ON and OFF periods may vary based on the presence of data to transmit and other factors. If the ON duration of a signal is significantly shorter than the minimum time period in which another DSL system can detect noise, the crosstalk may not be detected correctly. As a result, American National Standards Institute (ANSI) standard T1.417-2001 (Spectrum Management for Loop Transmission Systems), incorporated by reference herein, includes a requirement that the minimum transmission time for any transmission burst be at least 246 microseconds. See T1.417-2001 for additional background information on crosstalk, short-term stationary systems, and spectral compatibility requirements. It is possible that other regional requirements could be written that specify a different minimum transmission time.
One DSL method and apparatus meets the T1.417 requirement by adding a fixed number of padding symbols to empty frames to extend the length of those frames to the required, predefined minimum time. Such a DSL system is configured to operate in the United States and other regional communication systems that employ the T1.417 requirement.
In asynchronous transfer mode (ATM) cell based systems, transmissions can be extended in increments of one cell transmission period. However, this may be excessive relative to customer-specific requirements. The end-of-pad indication of the present invention allows a resolution to one symbol period even under poor communication conditions. Key system requirements such as scrambler initialization require accurate detection of the last symbol in the message. In modulation a false detection of the last symbol will cause a faulty upstream scrambler initialization and undetectable upstream data.
Other DSL systems are configured to operate in the other regional communication systems that employ standards and/or requirements that are different from the T1.417 requirement. Accordingly, equipment configured to operate under the T1.417 requirement may not be suitable for operation under other standards and/or requirements. When DSL equipment is to operate in regions having different standards and/or requirements, the equipment must be configured to operate using the standards and/or requirements of the region in which it is installed.
Having a variety of different DSL devices configured to operate under a variety of standards and/or requirements is undesirable because of the added cost to develop, manufacture and maintain separate DSL devices in a variety of locations, each location having different standards and/or requirements. Furthermore, the DSL equipment, once manufactured for a particular standard and/or requirement, is not interchangeable with DSL equipment used in another region having a different standard or requirement. Accordingly, if a DSL device, or one of its components, fails in the field, the failed device or component must be replaced using a device or component configured for operation under that particular standard and/or requirement. That is, a readily available “off-the-shelf” device or component cannot be used.